From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Eric Dumazet <edumazet@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
linux-kernel <linux-kernel@vger.kernel.org>,
Eric Dumazet <eric.dumazet@gmail.com>,
Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
netdev@vger.kernel.org, Marco Elver <elver@google.com>,
Uros Bizjak <ubizjak@gmail.com>
Subject: Re: [PATCH] locking/local_lock: Reduce local_[un]lock_nested_bh() overhead
Date: Wed, 11 Mar 2026 16:55:39 +0100 [thread overview]
Message-ID: <20260311155539.3Aikrlaz@linutronix.de> (raw)
In-Reply-To: <CANn89iKyPZUqmirYza5ewD2dBQfZCMunG771x7cMpUFkvw9x9w@mail.gmail.com>
On 2026-03-09 15:52:34 [+0100], Eric Dumazet wrote:
> > +void ericeric(void);
> > +void ericeric(void)
> > +{
> > + raw_cpu_read_long(this_cpu_off);
> > + raw_cpu_read_long(this_cpu_off);
> > +}
> >
> > I am guessing __raw_cpu_read() is forcing the asm ?
>
> Might be a clang issue. Oh well.
So the difference is that with gcc we have USE_X86_SEG_SUPPORT and with
llvm we don't. This leads to two asm statements with LLVM of which only
one is eliminated. This optimisation origins in commit ca4256348660c
("x86/percpu: Use C for percpu read/write accessors").
__seg_fs and __seg_gs is supported by LLVM but enabling it leads to tons
warnings and aborts later.
Is there something missing in LLVM? The generated code for
raw_cpu_read_long(this_cpu_off) looks fine.
Sebastian
next prev parent reply other threads:[~2026-03-11 15:55 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-03-09 12:20 [PATCH] locking/local_lock: Reduce local_[un]lock_nested_bh() overhead Eric Dumazet
2026-03-09 13:43 ` Peter Zijlstra
2026-03-09 13:49 ` Eric Dumazet
2026-03-09 14:05 ` Marco Elver
2026-03-09 14:11 ` Eric Dumazet
2026-03-09 14:03 ` Eric Dumazet
2026-03-09 14:18 ` Eric Dumazet
2026-03-09 14:52 ` Eric Dumazet
2026-03-11 15:55 ` Sebastian Andrzej Siewior [this message]
2026-03-11 16:32 ` Uros Bizjak
2026-03-11 16:45 ` Uros Bizjak
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